The invention relates to a continuous casting machine for the continuous casting of molten metal in particular molten steel into a cast product, comprising a mould in which the molten metal is poured through an exit port of pouring means, forming a bath of molten metal, and in which at least part of the metal is solidified, to a mould suitable for such continuous casting machine and to a method for the operation thereof.
A continuous casting machine as referred to in this specification may be any of the known continuous casting machines such as a conventional casting machine for casting slabs having a thickness of about 250 mm or a thin slab casting machine for casting slabs having a thickness of about 150 mm or less e.g. in the range 50-100 mm.
Although not restricted to thin slab casting machines, in particular in such machines where the velocity at which the metal enters into the mould is high, the problem of unstable and/or unsymmetrical flow of the molten metal in the mould occurs. Most commonly, molten metal is poured from a tundish into the mould through a submerged entry nozzle as pouring means connected to the tundish and reaching into the mould. The center line of the nozzle generally corresponds with the center line of the mould.
A continuous casting machine of the referred type is well known in the art e.g. from WO 95/20445. A mould and a nozzle suitable for such a continuous casting machine are known from WO 95/20443. A further embodiment of a nozzle is known from EP 0 685 282.
In practice it has shown that the molten metal after entering the mould forms recirculations of unequal magnitude and shape. In the case of a single exit port of the nozzle two recirculations develop in the vertical plane on either side of the nozzle: a smaller one and a large one. The recirculations extend to the meniscus and cause a disturbance thereof, which disturbance is different for each of the two recircula-tions. The heat transfer by the circulating molten metal to the casting powder, floating on the surface of the molten bath, and therefore the temperature of the casting powder is different for the two recirculations. Consequently the effect of the casting powder on the heat transfer of the molten metal to the chilled walls of the mould is not uniform. The same applies to the lubricating effect of the casting powder between the walls of the mould and the metal. The recirculations may also lead to entrapment of casting powder and other inclusions into the bath of molten metal. The resulting effect, apart from surface and bulk defects, is that the cast thin slab is not uniform in temperature and because of the unpredictability of the position of each of the recirculations, the temperature distribution is not predictable ultimately resulting in a non-uniform thickness, or in other words shape-defects, of the cast slab.
In modern steel making plants wherein in a continuous or semi-continuous process steel is cast, hot-rolled and in some cases ferriticly rolled, there is no or only a very limited possibility of correction of the shape of the cast slab. Therefore shape control in this type of plant is a particular problem.
Although the problem of unstable and unsymmetrical flow in the mould has been elucidated with regard to thin slab casting, the problem also occurs in thick slab casting machines.
A direction in which in the prior art a solution was sought was the shape of the nozzle and of the exit ports thereof. Numerous proposals for the shape of the exit port, its angle relation to the longitudinal axis of the nozzle and the shape of the bottom of the nozzle were made. In thin slabs this necessitated a funnel shape of the mould.
Following this direction has not led to a satisfactory solution of the above-mentioned problems, in particular not to a solution suitable for the various casting conditions connected with various steel grades and sizes of the cast product.
An object of the invention is to provide a continuous casting machine with which these problems can be obviated or at least largely reduced and with which also other advantages can be obtained.
This object is reached with a continuous casting machine that is characterized in that, it is provided with control means for controlling and/or steering the flow of molten metal and operative on the molten metal after entering the mould such that the flow pattern of the molten metal in the mould is basically symmetrical with respect to at least one plane of symmetry of the mould.